Figuring out FOV and Angle for a Reference? How to use a Reference Image?
Images as Planes are textured plane geometry, so they can only be viewed in Texture, Material (LookDev), or Render mode.  Use the Empty/ImageType for your background image.  Select your camera view in the viewport. (numpad 0).  Then Add->Image->Background (in 2.8).   This will create the ImagePlane square to your camera view.  In the Properties -> Object Data choose the Back depth or Default Depth.  Just remember that you cannot render the background image in your final render, but you can render it in Viewport Render from the View menu.